Lighting buildings is estimated to require at least 20 percent of the global electricity consumption. At least in part because of this, the use of energy inefficient light bulbs, especially incandescent lighting, is typically being phased out legislatively and through promotion of more efficient lighting means.
Compact fluorescent light bulbs (CFLs) represent a more energy efficient alternative. However, CFLs are criticized for containing mercury, a very toxic and hazardous substance that could be released in home upon breakage. CFLs also face criticism that they do not generate the same white light tone as incandescent light bulbs, which may negatively impact consumer sentiment.
The use of LEDs is known for providing lighting systems with improved characteristics including: lower power consumption, extended lifetime, smaller size, and improved durability and reliability. LEDs generally use up to 90 percent less energy than traditional incandescent bulbs and can have a useful lifetime of up to 50,000 hours. LEDs also have important applications in other market segments, including auto industry, displays and TV backlights, for which the quality of illumination (brightness and color purity) is important, as is the cost and efficiency.
A further alternative may be Organic Light Emitting Diodes (OLEDs). OLEDs generally have simpler design than inorganic LEDs, but involve a complex synthesis procedure and contain expensive heavy or rare earth metals (platinum, iridium etc.). As well, organic or organometallic LEDs are generally less stable and less efficient than inorganic LEDs.
Light bulbs that use LEDs do not contain mercury and generally a much longer lifespan than the CFLs. The energy efficiency of a LED light bulb is superior to a traditional incandescent light bulb and at least as efficient, if not more so, than a CFL.
However, LED lighting sources and materials (such as, molecules or semiconductors) generally emit light in a narrow range of the visible spectrum, making the design of white light emitters very challenging and costly. Four conventional approaches for creating white light emitting diodes (LEDs) are described here.
In the first conventional approach, light emitting diodes with blue, green and red outputs have been combined in a light emitting structure to give an illusion of white light. There are several disadvantages to this approach, including the difficulty in making optimal green LEDs, and a very high design complexity and cost of manufacturing.
In the second conventional approach, white LEDs have been created by coating ultraviolet (UV) or blue LEDs (usually based on III-V semiconductors—gallium, gallium-indium or aluminum nitride) with one or more inorganic phosphors emitting complementary colors. If a blue LED is used, then a part of the emitted light is converted by using a phosphor material. Specifically, white LEDs have been created by blue LEDs with doped phosphors, such as Ce:YAG (cerium-doped yttrium aluminum garnet). In doped phosphors the dopant ions emit complementary colors to blue, for example yellow or orange, giving off quasi-white light upon illumination with a blue LED. In this approach it is difficult to find an appropriate dopant-host combination and to control the doping process in a reproducible way, generally resulting in low purity white light emissions. Furthermore, the difficulty of adjusting the fraction of LED emission which is absorbed by the phosphor to give off a white light appearance also contributes to lower quality of the obtained white light source. This approach generally requires the use of materials containing rare earth elements which are becoming increasingly scarce and expensive. As well, this method entails complex design requirements, and can result in lack of homogeneity (impurity) of white light illumination as a consequence of using multiple phosphors, or LEDs, and a single phosphor emission to produce white light.
In the third conventional approach, white LEDs have been created by using organic molecule-based electroluminescence. The most common approach within this strategy is coating an organic molecule-based blue electroluminescence device with multiple layers of organic molecules emitting different colors. This approach requires complex processing and generally results in large amounts of wasted organic material, resulting in relatively high fabrication cost.
In the fourth conventional approach, a blend of multiple organic molecule emitters is coated as a single layer in an electroluminescent device. This approach is more cost-effective, but results in low quality (impure) white light emission. Organic LEDs generally have a simpler design than inorganic LEDs, but involve complex synthetic procedures, and often also contain expensive heavy metals (platinum, iridium etc.) in case of organometallic emitters. Furthermore, organic LEDs are generally less stable than inorganic LEDs.
In general, the construction of white LEDs is challenging and costly because of the difficulties in obtaining multi-color emission in the necessary proportions.
There is therefore a need for an improved white light emitting material or LED and a method of synthesizing or fabricating white light emitting materials that overcomes at least some of the shortcomings of conventional LEDs and methods.